A group of masked men burned down a border crossing manned by Kosovan and UN police as Serb protests against Kosovo's declaration of independence turned violent.

A Reuters witness said the police did not intervene as a mob set fire to the Jarinje post on the boundary between Kosovo and Serbia known as Gate 1.

Kosovan police took shelter in a tunnel and called for help from Nato peacekeepers as more than 1,000 protesters tried to tear down a second crossing near Zubin Potok known as Gate 2.

"We asked Nato to send a helicopter to evacuate our officers," a police source told Reuters in the capital, Pristina.

The flare-up in northern Kosovo, where most of the 120,000 Kosovo Serbs live, came as the EU foreign policy chief, Javier Solana, made a surprise visit for talks with leaders from the former Serbian province.

Meanwhile, the US president, George Bush, sought to shore up international support for the fledgling state amid Russian and Chinese opposition to Kosovo's independence.

"The independence of Kosovo is an historic step for the Balkans region," Bush said in Tanzania before flying to Rwanda during an African tour. "It presents an opportunity to move beyond the conflicts of the past and toward a future of freedom and stability and peace."

As well as the attack on the border post, explosions shook northern Kosovo and in Belgrade the US embassy temporarily closed its doors after two days of riots targeting western missions.

In the divided town of Mitrovica, one of the blasts damaged several cars near a UN building, while two hand grenade explosions hit deserted and already destroyed homes belonging to ethnic Albanians who fled after the 1999 war. No injuries were reported.

Nato peacekeepers deployed explosives experts to search for devices allegedly planted in a high school in the Serb village of Laplje Selo, just south of Pristina. Students were evacuated after the school director received a call saying a hand grenade was about to explode.

Russia, a strong supporter of Serbia, reiterated its warnings about what it believes to be a dangerous precedent with Kosovo's declaration of independence.

The Russian foreign minister, Sergei Lavrov, warned the US secretary of state, Condoleezza Rice, that Kosovo's declaration of independence endangered international stability, the foreign ministry said.

The ministry said in a statement Lavrov had discussed Kosovo with Rice in a telephone conversation initiated by Washington.

"We underlined the dangerous consequences of such a step, which is fraught with the destruction of the principles of peace and order and international stability which have been developed over decades," the statement said.

Until its declaration of independence on Sunday, Kosovo had been under UN supervision since 1999, when Nato bombing forced the withdrawal of Serb forces engaged in a campaign of ethnic cleansing.

The US and major EU countries – Britain, France, Italy and Germany – have said they would recognise Kosovo. Seventeen of the EU's 27 members are expected to recognise the fledgling state in the next few days, but Spain, Cyprus and Slovakia oppose independence for Kosovo as they face their own separatist problems.